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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Chemistry".

Deadline for manuscript submissions: closed (10 February 2018) | Viewed by 98777

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Prof. Dr. John C. Walton

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EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK
Interests: radical chemistry; organic synthetic methods; free-radical rearrangements; photoredox catalysis; oxime derivatives; EPR spectroscopy; DFT applications; enhanced acidity of radicals
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Dear Colleagues,

Radicals play an astonishing variety of roles in an amazingly diverse range of sciences and technologies. Our understanding of the almost unlimited flexibility of their structures and the huge breadth of their activities has expanded wonderfully in the last few years. Notable new advances include: The burgeoning exploitation of photoredox catalysis in mild synthetic procedures, boron-containing radicals in syntheses, oxime derivatives as radical precursors, radical cascade reactions, novel controlled/living radical mediated polymerization methods, double spin labelling for EPR distance measurements in biopolymers and organic super electron donors. Radical-mediated syntheses are steadily taking their place alongside more traditional nucleophile/electrophile preparative procedures. In fact, radical-mediated preparations frequently enable tedious protection/deprotection steps to be dispensed with and this, coupled with the neutral conditions and absence of harsh acidic/basic reagents, makes their use particularly attractive. Radical reactivity depends strongly on the underlying thermodynamics. Key thermodynamic parameters have been obtained for many model radicals and archetype radical clocks are available for assessing reactivity. These tools, supplemented and augmented by DFT computational methods, ensure that synthetic planning is comparatively easy and that mechanisms can be rationally established. Furthermore, radical intermediates can often be elegantly characterized and monitored by EPR spectroscopic methods. Persistent radicals are finding more and more uses in both biological and materials sciences. The aim of this Special Issue is to review and showcase recent research across the whole field. Papers and review articles are welcomed in the heartland areas of radical-based synthesis and physical organic chemistry, as well as in all the newly-developing fields.

Prof. Dr. John C. Walton
Guest Editor

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Keywords

Reactive intermediates
Radical-mediated synthetic methods
Radical reagents
Radical cyclizations
Radical rearrangements
Radical kinetics and mechanisms
Redox properties of radicals
Thermochemistry of radicals
Photoredox catalysis
Nitroxides
Spin trapping and spin labelling
Autoxidations
Radical mediated polymerizations
Applications of EPR spectroscopy

Related Special Issue

Advances in Radical Chemistry in Molecules (3 articles)

Published Papers (13 papers)

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Research

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16 pages, 4206 KiB

Open AccessArticle

Visible-Light, Iodine-Promoted Formation of N-Sulfonyl Imines and N-Alkylsulfonamides from Aldehydes and Hypervalent Iodine Reagents

by Megan D. Hopkins, Zachary C. Brandeburg, Andrew J. Hanson and Angus A. Lamar

Molecules 2018, 23(8), 1838; https://doi.org/10.3390/molecules23081838 - 24 Jul 2018

Cited by 12 | Viewed by 6939

Abstract

Alternative synthetic methodology for the direct installation of sulfonamide functionality is a highly desirable goal within the domain of drug discovery and development. The formation of synthetically valuable N-sulfonyl imines from a range of aldehydes, sulfonamides, and PhI(OAc)₂ under practical and mild reaction conditions has been developed. According to mechanistic studies described within, the reaction proceeds through an initial step involving a radical initiator (generated either by visible-light or heat) to activate the reacting substrates. The reaction provides a synthetically useful and operationally simple, relatively mild alternative to the traditional formation of N-sulfonyl imines that utilizes stable, widely available reagents. Full article

(This article belongs to the Special Issue Radical Chemistry)

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16 pages, 5040 KiB

Open AccessFeature PaperArticle

KO^tBu as a Single Electron Donor? Revisiting the Halogenation of Alkanes with CBr₄ and CCl₄

by Katie J. Emery, Allan Young, J. Norman Arokianathar, Tell Tuttle and John A. Murphy

Molecules 2018, 23(5), 1055; https://doi.org/10.3390/molecules23051055 - 01 May 2018

Cited by 8 | Viewed by 13841

Abstract

The search for reactions where KO^tBu and other tert-alkoxides might behave as single electron donors led us to explore their reactions with tetrahalomethanes, CX₄, in the presence of adamantane. We recently reported the halogenation of adamantane under these conditions. These reactions appeared to mirror the analogous known reaction of NaOH with CBr₄ under phase-transfer conditions, where initiation features single electron transfer from a hydroxide ion to CBr₄. We now report evidence from experimental and computational studies that KO^tBu and other alkoxide reagents do not go through an analogous electron transfer. Rather, the alkoxides form hypohalites upon reacting with CBr₄ or CCl₄, and homolytic decomposition of appropriate hypohalites initiates the halogenation of adamantane. Full article

(This article belongs to the Special Issue Radical Chemistry)

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11 pages, 3565 KiB

Open AccessArticle

Porphyrin Co(III)-Nitrene Radical Mediated Pathway for Synthesis of o-Aminoazobenzenes

by Monalisa Goswami and Bas De Bruin

Molecules 2018, 23(5), 1052; https://doi.org/10.3390/molecules23051052 - 01 May 2018

Cited by 9 | Viewed by 5485

Abstract

Azobenzenes are versatile compounds with a range of applications, including dyes and pigments, food additives, indicators, radical reaction initiators, molecular switches, etc. In this context, we report a general method for synthesizing o-aminoazobenzenes using the commercially available cobalt(II) tetraphenyl porphyrin [Co^II(TPP)]. The net reaction is a formal dimerization of two phenyl azides with concomitant loss of two molecules of dinitrogen. The most commonly used methodology to synthesize azobenzenes is based on the initial diazotization of an aromatic primary amine at low temperatures, which then reacts with an electron rich aromatic nucleophile. As such, this limits the synthesis of azobenzenes with an amine functionality. In contrast, the method we report here relies heavily on the o-amine moiety and retains it in the product. The reaction is metal catalyzed and proceeds through a porphyrin Co(III)-nitrene radical intermediate, which is known to form on activation of organic azides at the cobalt center. The synthesized o-aminoazobenzenes are bathochromatically shifted, as compared to azobenzenes without amine substituents. Based on the crystal structure of one of the products, strong H-bonding between the N-atom of the azo functionality and the H of the NH₂ substituent is shown to stabilize the trans isomeric form of the product. The NH₂ substituents offers possibilities for further functionalization of the synthesized azo compounds. Full article

(This article belongs to the Special Issue Radical Chemistry)

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11 pages, 646 KiB

Open AccessFeature PaperArticle

Radical-Mediated Reactions of α-Bromo Aluminium Thioacetals, α-Bromothioesters, and Xanthates for Thiolactone Synthesis

by Ruairí O. McCourt, Fabrice Dénès and Eoin M. Scanlan

Molecules 2018, 23(4), 897; https://doi.org/10.3390/molecules23040897 - 13 Apr 2018

Cited by 5 | Viewed by 5320

Abstract

Thiolactones have attracted considerable attention in recent years as bioactive natural products, lead compounds for drug discovery, molecular probes, and reagents for polymerisation. We have investigated radical-mediated C-C bond forming reactions as a strategy for thiolactone synthesis. Cyclisation of an α-bromo aluminium thioacetal was investigated under radical conditions. It was found that at low temperature, a radical fragmentation and rearrangement process occurs. A putative reaction mechanism involving a previously unreported aluminium templated thiol-ene step for the rearrangement process is presented. Cyclisation reactions of α-bromo thioesters and α-xanthate thioesters under radical mediated conditions furnished the desired thiolactones in moderate yields. Full article

(This article belongs to the Special Issue Radical Chemistry)

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16 pages, 3208 KiB

Open AccessArticle

A Visible Light-Driven Minisci-Type Reaction with N-Hydroxyphthalimide Esters

by Lisa Marie Kammer, Aliyaah Rahman and Till Opatz

Molecules 2018, 23(4), 764; https://doi.org/10.3390/molecules23040764 - 27 Mar 2018

Cited by 44 | Viewed by 12757

Abstract

A visible light-promoted protocol for the redox-neutral coupling of N-hydroxyphthalimide esters with different N-heterocyclic compounds is described. The reaction proceeds through an alkyl radical intermediate generated by reductive decarboxylation of N-hydroxyphthalimide esters. In contrast to the original Minisci protocol, polyalkylation can largely be avoided. Mechanistic investigations revealed a radical chain mechanism which in some cases can proceed even if no photocatalyst is added. This valuable and functional group-tolerant reaction produces substituted heterocycles in moderate to excellent yield. The use of inexpensive starting materials and LEDs as the light source are key features of this C–C bond formation. Full article

(This article belongs to the Special Issue Radical Chemistry)

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18 pages, 4960 KiB

Open AccessArticle

Synthesis of Nanometer Sized Bis- and Tris-trityl Model Compounds with Different Extent of Spin–Spin Coupling

by J. Jacques Jassoy, Andreas Meyer, Sebastian Spicher, Christine Wuebben and Olav Schiemann

Molecules 2018, 23(3), 682; https://doi.org/10.3390/molecules23030682 - 17 Mar 2018

Cited by 18 | Viewed by 5804

Abstract

Tris(2,3,5,6-tetrathiaaryl)methyl radicals, so-called trityl radicals, are emerging as spin labels for distance measurements in biological systems based on Electron Paramagnetic Resonance (EPR). Here, the synthesis and characterization of rigid model systems carrying either two or three trityl moieties is reported. The monofunctionalized trityl radicals are connected to the molecular bridging scaffold via an esterification reaction employing the Mukaiyama reagent 2-chloro-methylpyridinium iodide. The bis- and tris-trityl compounds exhibit different inter-spin distances, strength of electron–electron exchange and dipolar coupling and can give rise to multi-spin effects. They are to serve as benchmark systems in comparing EPR distance measurement methods. Full article

(This article belongs to the Special Issue Radical Chemistry)

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14 pages, 5751 KiB

Open AccessArticle

Mechanistic Insight into the Degradation of Nitrosamines via Aqueous-Phase UV Photolysis or a UV-Based Advanced Oxidation Process: Quantum Mechanical Calculations

by Daisuke Minakata and Erica Coscarelli

Molecules 2018, 23(3), 539; https://doi.org/10.3390/molecules23030539 - 28 Feb 2018

Cited by 14 | Viewed by 5612

Abstract

Nitrosamines are a group of carcinogenic chemicals that are present in aquatic environments that result from byproducts of industrial processes and disinfection products. As indirect and direct potable reuse increase, the presence of trace nitrosamines presents challenges to water infrastructures that incorporate effluent from wastewater treatment. Ultraviolet (UV) photolysis or UV-based advanced oxidation processes that produce highly reactive hydroxyl radicals are promising technologies to remove nitrosamines from water. However, complex reaction mechanisms involving radicals limit our understandings of the elementary reaction pathways embedded in the overall reactions identified experimentally. In this study, we perform quantum mechanical calculations to identify the hydroxyl radical-induced initial elementary reactions with N-nitrosodimethylamine (NDMA), N-nitrosomethylethylamine, and N-nitrosomethylbutylamine. We also investigate the UV-induced NDMA degradation mechanisms. Our calculations reveal that the alkyl side chains of nitrosamine affect the reaction mechanism of hydroxyl radicals with each nitrosamine investigated in this study. Nitrosamines with one- or two-carbon alkyl chains caused the delocalization of the electron density, leading to slower subsequent degradation. Additionally, three major initial elementary reactions and the subsequent radical-involved reaction pathways are identified in the UV-induced NDMA degradation process. This study provides mechanistic insight into the elementary reaction pathways, and a future study will combine these results with the kinetic information to predict the time-dependent concentration profiles of nitrosamines and their transformation products. Full article

(This article belongs to the Special Issue Radical Chemistry)

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13 pages, 3320 KiB

Open AccessArticle

Radical Chemistry in a Femtosecond Laser Plasma: Photochemical Reduction of Ag⁺ in Liquid Ammonia Solution

by Victoria Kathryn Meader, Mallory G. John, Laysa M. Frias Batista, Syeda Ahsan and Katharine Moore Tibbetts

Molecules 2018, 23(3), 532; https://doi.org/10.3390/molecules23030532 - 27 Feb 2018

Cited by 32 | Viewed by 6910

Abstract

Plasmas with dense concentrations of reactive species such as hydrated electrons and hydroxyl radicals are generated from focusing intense femtosecond laser pulses into aqueous media. These radical species can reduce metal ions such as Au³⁺ to form metal nanoparticles (NPs). However, the formation of H₂O₂ by the recombination of hydroxyl radicals inhibits the reduction of Ag⁺ through back-oxidation. This work has explored the control of hydroxyl radical chemistry in a femtosecond laser-generated plasma through the addition of liquid ammonia. The irradiation of liquid ammonia solutions resulted in a reaction between NH₃ and OH·, forming peroxynitrite and ONOO⁻, and significantly reducing the amount of H₂O₂ generated. Varying the liquid ammonia concentration controlled the Ag⁺ reduction rate, forming 12.7 ± 4.9 nm silver nanoparticles at the optimal ammonia concentration. The photochemical mechanisms underlying peroxynitrite formation and Ag⁺ reduction are discussed. Full article

(This article belongs to the Special Issue Radical Chemistry)

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14 pages, 3962 KiB

Open AccessFeature PaperArticle

Microhydration and the Enhanced Acidity of Free Radicals

by John C. Walton

Molecules 2018, 23(2), 423; https://doi.org/10.3390/molecules23020423 - 14 Feb 2018

Cited by 7 | Viewed by 4621

Abstract

Recent theoretical research employing a continuum solvent model predicted that radical centers would enhance the acidity (RED-shift) of certain proton-donor molecules. Microhydration studies employing a DFT method are reported here with the aim of establishing the effect of the solvent micro-structure on the acidity of radicals with and without RED-shifts. Microhydration cluster structures were obtained for carboxyl, carboxy-ethynyl, carboxy-methyl, and hydroperoxyl radicals. The numbers of water molecules needed to induce spontaneous ionization were determined. The hydration clusters formed primarily round the CO₂ units of the carboxylate-containing radicals. Only 4 or 5 water molecules were needed to induce ionization of carboxyl and carboxy-ethynyl radicals, thus corroborating their large RED-shifts. Full article

(This article belongs to the Special Issue Radical Chemistry)

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4354 KiB

Open AccessArticle

Novel Carbazole Skeleton-Based Photoinitiators for LED Polymerization and LED Projector 3D Printing

by Assi Al Mousawi, Patxi Garra, Frédéric Dumur, Thanh-Tuan Bui, Fabrice Goubard, Joumana Toufaily, Tayssir Hamieh, Bernadette Graff, Didier Gigmes, Jean Pierre Fouassier and Jacques Lalevée

Molecules 2017, 22(12), 2143; https://doi.org/10.3390/molecules22122143 - 04 Dec 2017

Cited by 57 | Viewed by 7828

Abstract

Radical chemistry is a very convenient way to produce polymer materials. Here, an application of a particular photoinduced radical chemistry is illustrated. Seven new carbazole derivatives Cd1–Cd7 are incorporated and proposed as high performance near-UV photoinitiators for both the free radical polymerization (FRP) of (meth)acrylates and the cationic polymerization (CP) of epoxides utilizing Light Emitting Diodes LEDs @405 nm. Excellent polymerization-initiating abilities are found and high final reactive function conversions are obtained. Interestingly, these new derivatives display much better near-UV polymerization-initiating abilities compared to a reference UV absorbing carbazole (CARET 9H-carbazole-9-ethanol) demonstrating that the new substituents have good ability to red shift the absorption of the proposed photoinitiators. All the more strikingly, in combination with iodonium salt, Cd1–Cd7 are likewise preferred as cationic photoinitiators over the notable photoinitiator bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) for mild irradiation conditions featuring their remarkable reactivity. In particular their utilization in the preparation of new cationic resins for LED projector 3D printing is envisioned. A full picture of the included photochemical mechanisms is given. Full article

(This article belongs to the Special Issue Radical Chemistry)

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3641 KiB

Open AccessArticle

Theoretical Study of ClOO + NO Reaction: Mechanism and Kinetics

by Nan-Nan Wu, Shun-Li Ou-Yang and Liang Li

Molecules 2017, 22(12), 2121; https://doi.org/10.3390/molecules22122121 - 01 Dec 2017

Cited by 2 | Viewed by 3794

Abstract

Theoretical investigations are performed on mechanism and kinetics of the reaction of halogen peroxy radical ClOO with NO radical. The electronic structure information for both of the singlet and triplet potential energy surfaces (PESs) is obtained at the MP2/6-311 + G(2df) level of theory, and the single-point energies are refined by the CCSD(T)/6-311 + G(2df) level. The rate constants for various product channels of the reaction in the pressure range of 1-7600 Torr are predicted. The main results are as follows: On the singlet surface, the addition-elimination mechanism is the most important. First, the N atom of the NO radical can attack the O atom of the ClOO radical to form an energy-riched intermediate IM1 ClOONOtp (21.3 kcal/mol) barrierlessly, then IM1 could isomerizes to IM2 ClOONOcp (22.1 kcal/mol) via a low energy barrier. Both IM1 and IM2 can dissociate to the primary product P₁ ClNO + ¹O₂ and the secondary product P₂ ClO + NO₂. On the triplet surface, the direct Cl-abstraction reaction is the most feasible pathway. The Cl-abstraction can take place via a van der Waals complex, ³IM1 ONClOO (4.1 kcal/mol), then it fragments readily to give P₁’ ClNO + ³O₂ with a small barrier. The kinetic calculations show that at low temperatures, the singlet bimolecular product P₁ is the primary product, while at high temperatures, the triplet product P₁’ becomes the primary one; only at high pressures and low temperatures, the unimolecular products IM1 and IM2 can be found with quite small yields. At experimentally measured temperature 213 K, ClNO is the primary product in the whole pressure range, which is consistent with the previous experiment. The present study may be useful for further experimental studies for the title reaction. Full article

(This article belongs to the Special Issue Radical Chemistry)

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Review

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21 pages, 8166 KiB

Open AccessFeature PaperReview

Radical Polymerization of Alkyl 2-Cyanoacrylates

by Cormac Duffy, Per B. Zetterlund and Fawaz Aldabbagh

Molecules 2018, 23(2), 465; https://doi.org/10.3390/molecules23020465 - 20 Feb 2018

Cited by 40 | Viewed by 10377

Abstract

Cyanoacrylates (CAs) are well-known fast-setting adhesives, which are sold as liquids in the presence of stabilizers. Rapid anionic polymerization on exposure to surface moisture is responsible for instant adhesion. The more difficult, but synthetically more useful radical polymerization is only possible under acidic conditions. Recommendations on the handling of CAs and the resulting polymers are provided herein. In this review article, after a general description of monomer and polymer properties, radical homo- and copolymerization studies are described, along with an overview of nanoparticle preparations. A summary of our recently reported radical polymerization of CAs, using reversible addition-fragmentation chain transfer (RAFT) polymerization, is provided. Full article

(This article belongs to the Special Issue Radical Chemistry)

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15 pages, 3293 KiB

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Ultrafast Chemistry of Water Radical Cation, H₂O^•+, in Aqueous Solutions

by Jun Ma, Furong Wang and Mehran Mostafavi

Molecules 2018, 23(2), 244; https://doi.org/10.3390/molecules23020244 - 26 Jan 2018

Cited by 67 | Viewed by 7738

Abstract

Oxidation reactions by radicals constitute a very important class of chemical reactions in solution. Radiation Chemistry methods allow producing, in a controlled way, very reactive oxidizing radicals, such as OH^•, CO₃^•–, NO₃^•, SO₄^•–, and N₃^•. Although the radical cation of water, H₂O^•+, with a very short lifetime (shorter than 1 ps) is the precursor of these radicals in aqueous solutions, its chemistry is usually known to be limited to the reaction of proton transfer by forming OH^• radical. Herein, we stress situations where H₂O^•+ undergoes electron transfer reaction in competition with proton transfer. Full article

(This article belongs to the Special Issue Radical Chemistry)

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